You like to win money, so you click on the submit button. In the process, you have unintentionally transferred $100 to
a malicious user. This happens because, while the evil website cannot see your cookies, the cookies associated with your
bank are still sent along with the request.

Worst yet, this whole process could have been automated using JavaScript. This means you didn't even need to click on the
button. So how do we protect ourselves from such attacks?

13.2 Synchronizer Token Pattern

The issue is that the HTTP request from the bank's website and the request from the evil website are exactly the same. This
means there is no way to reject requests coming from the evil website and allow requests coming from the bank's website. To
protect against CSRF attacks we need to ensure there is something in the request that the evil site is unable to provide.

One solution is to use the
Synchronizer
Token Pattern. This solution is to ensure that each request requires, in addition to our session cookie, a randomly
generated token as an HTTP parameter. When a request is submitted, the server must look up the expected value for the parameter
and compare it against the actual value in the request. If the values do not match, the request should fail.

We can relax the expectations to only require the token for each HTTP request that updates state. This can be safely done
since the same origin policy ensures the evil site cannot read the response. Additionally, we do not want to include the random
token in HTTP GET as this can cause the tokens to be leaked.

Let's take a look at how our example would change. Assume the randomly generated token is present in an HTTP parameter named
_csrf. For example, the request to transfer money would look like this:

You will notice that we added the _csrf parameter with a random value. Now the evil website will not be able to guess the
correct value for the _csrf parameter (which must be explicitly provided on the evil website) and the transfer will fail when the
server compares the actual token to the expected token.

13.3 When to use CSRF protection

When you use CSRF protection? Our recommendation is to use CSRF protection for any request that could be processed by a browser by normal users. If you are only creating
a service that is used by non-browser clients, you will likely want to disable CSRF protection.

13.3.1 CSRF protection and JSON

A common question is, but do I need to protect JSON requests made by javascript? The short answer is, it depends. However, you must be very careful as there
are CSRF exploits that can impact JSON requests. For example, a malicious user can create a
CSRF with JSON using the following form:

If an application were not validating the Content-Type, then it would be exposed to this exploit. Depending on the setup, a Spring MVC application that validates the
Content-Type could still be exploited by updating the URL suffix to end with ".json" as shown below:

13.3.2 CSRF and Stateless Browser Applications

What if my application is stateless? That doesn't necessarily mean you are protected. In fact, if a user does not need to perform any actions in the web browser for a given
request, they are likely still vulnerable to CSRF attacks.

For example, consider an application uses a custom cookie that contains all the state within it for authentication instead of the JSESSIONID. When the CSRF attack is made
the custom cookie will be sent with the request in the same manner that the JSESSIONID cookie was sent in our previous example.

User's using basic authentication are also vulnerable to CSRF attacks since the browser will automatically include the username password in any requests in the same manner that
the JSESSIONID cookie was sent in our previous example.

13.4 Using Spring Security CSRF Protection

So what are the steps necessary to use Spring Security's to protect our site against CSRF attacks? The steps to using Spring
Security's CSRF protection are outlined below:

13.4.1 Use proper HTTP verbs

The first step to protecting against CSRF attacks is to ensure your website uses proper HTTP verbs. Specifically, before Spring
Security's CSRF support can be of use, you need to be certain that your application is using PATCH, POST, PUT, and/or DELETE for anything
that modifies state.

This is not a limitation of Spring Security's support, but instead a general requirement for proper CSRF prevention. The reason is that
including private information in an HTTP GET can cause the information to be leaked. See
RFC 2616 Section 15.1.3 Encoding Sensitive Information in URI's for
general guidance on using POST instead of GET for sensitive information.

13.4.2 Configure CSRF Protection

The next step is to include Spring Security's CSRF protection within your application. Some frameworks handle invalid CSRF tokens by invaliding the user's
session, but this causes its own problems. Instead by default Spring Security's CSRF protection will produce an HTTP 403 access denied.
This can be customized by configuring the AccessDeniedHandler to process InvalidCsrfTokenException
differently.

For passivity reasons, if you are using the XML configuration, CSRF protection must be explicitly enabled using the <csrf> element. Refer to the
<csrf> element's documentation for additional customizations.

CSRF protection is enabled by default with Java configuration. If you would like to disable CSRF, the corresponding Java configuration can be
seen below. Refer to the Javadoc of csrf() for additional customizations in how CSRF protection is configured.

13.4.3 Include the CSRF Token

Form Submissions

The last step is to ensure that you include the CSRF token in all PATCH, POST, PUT, and DELETE methods. This can be done using
the _csrf request attribute to obtain the current CsrfToken. An example of doing this with a JSP is shown below:

If you are using Spring MVC <form:form> tag, the CsrfToken is automatically included for you using the CsrfRequestDataValueProcessor.

Ajax and JSON Requests

If you using JSON, then it is not possible to submit the CSRF token within an HTTP parameter. Instead you can submit the token within a HTTP header.
A typical pattern would be to include the CSRF token within your meta tags. An example with a JSP is shown below:

As a alternative to jQuery, we recommend using cujoJS’s rest.js. rest.js provides
advanced support for working with HTTP request and responses in RESTful ways. A core capability is the ability to contextualize the HTTP client adding behavior as needed by
chaining interceptors on to the client.

The configured client can be shared with any component of the application that needs to make a request to the CSRF protected resource. One significant different between rest.js
and jQuery is that only requests made with the configured client will contain the CSRF token, vs jQuery where all requests will include the token. The ability
to scope which requests receive the token helps guard against leaking the CSRF token to a third party. Please refer to the
rest.js reference documentation for more information on rest.js.

13.5 CSRF Caveats

There are a few caveats when implementing CSRF.

13.5.1 Timeouts

One issue is that the expected CSRF token is stored in the HttpSession, so as soon as the HttpSession expires your configured
AccessDeniedHandler will receive a InvalidCsrfTokenException. If you are using the default
AccessDeniedHandler, the browser will get an HTTP 403 and display a poor error message.

Note

One might ask why the expected CsrfToken isn't stored in a cookie. This is because there are known exploits in which headers
(i.e. specify the cookies) can be set by another domain. This is the same reason Ruby on Rails
no longer skips CSRF checks when the header X-Requested-With
is present. See this webappsec.org thread
for details on how to perform the exploit. Another disadvantage is that by removing the state (i.e. the timeout) you lose the ability
to forcibly terminate the token if something got compromised.

A simple way to mitigate an active user experiencing a timeout is to have some JavaScript that lets the user know their session is about to expire.
The user can click a button to continue and refresh the session.

Alternatively, specifying a custom AccessDeniedHandler allows you to process the InvalidCsrfTokenException
anyway you like. For an example of how to customize the AccessDeniedHandler refer to the provided links for both
xml and
Java
configuration.

13.5.2 Logging In

In order to protect against forging log in requests the log in form should be protected against CSRF attacks too. Since the CsrfToken is stored in
HttpSession, this means an HttpSession will be created as soon as CsrfToken token attribute is accessed. While this sounds bad in
a RESTful / stateless architecture the reality is that state is necessary to implement practical security. Without state, we have nothing we can do if a token is
compromised. Practically speaking, the CSRF token is quite small in size and should have a negligible impact on our architecture.

13.5.3 Logging Out

Adding CSRF will update the LogoutFilter to only use HTTP POST. This ensures that log out requires a CSRF token and that a malicious user cannot forcibly
log out your users.

One approach is to use a form for log out. If you really want a link, you can use JavaScript to have the link perform a POST (i.e. maybe on a hidden form). For
browsers with JavaScript that is disabled, you can optionally have the link take the user to a log out confirmation page that will perform the POST.

13.5.4 Multipart (file upload)

There are two options to using CSRF protection with multipart/form-data. Each option has its tradeoffs.

Placing MultipartFilter before Spring Security

The first option is to ensure that the MultipartFilter is specified before the Spring
Security filter. Specifying the MultipartFilter after the Spring Security filter means that there is no authorization for invoking the
MultipartFilter which means anyone can place temporary files on your server. However, only authorized users will be able to submit a File that is processed
by your application. In general, this is the recommended approach because the temporary file upload should have a negligble impact on most servers.

To ensure MultipartFilter is specified before the Spring Security filter with java configuration, users can override beforeSpringSecurityFilterChain as
shown below:

To ensure MultipartFilter is specified before the Spring Security filter with XML configuration, users can ensure the <filter-mapping> element
of the MultipartFilter is placed before the springSecurityFilterChain within the web.xml as shown below:

Include CSRF token in action

If allowing unauthorized users to upload temporariy files is not acceptable, an alternative is to place the MultipartFilter after the Spring Security
filter and include the CSRF as a query parameter in the action attribute of the form. An example with a jsp is shown below

13.5.5 HiddenHttpMethodFilter

The HiddenHttpMethodFilter should be placed before the Spring Security filter. In general this is true, but it could have additional implications when
protecting against CSRF attacks.

Note that the HiddenHttpMethodFilter only overrides the HTTP method on a POST, so this is actually unlikely to cause any real problems. However, it is still
best practice to ensure it is placed before Spring Security's filters.

13.6 Overriding Defaults

Spring Security's goal is to provide defaults that protect your users from exploits. This does not mean that you are forced to accept all of its defaults.

For example, you can provide a custom CsrfTokenRepository to override the way in which the CsrfToken is stored.

You can also specify a custom RequestMatcher to determine which requests are protected by CSRF (i.e. perhaps you don't care if log out is exploited). In short, if
Spring Security's CSRF protection doesn't behave exactly as you want it, you are able to customize the behavior. Refer to the <csrf>
documentation for details on how to make these customizations with XML and the CsrfConfigurer javadoc for details on how to make these
customizations when using Java configuration.